There are five known low molecular weight basic proteins in myelin. The 14K, 17K, 18.5K and 21.5K basic proteins of CNS myelin are also found in PNS myelin and are all inter-related by immunological and primary structure criteria. The sequences of the 18.5K and 14K are known, as is the sequence of the fifth basic protein, the P2 protein of PNS myelin. Available data indicates that there is no sequence homology or immunological cross reactivity between the P2 protein and the other four low molecular weight basic proteins. Preliminary evidence suggests that the difference between the sequence of the 14K and 17K and between the 18.5K and 21.5K is a single insertion of 25-30 amino acids which has the same sequence in both the 17K and 21.5K protein. In order to complete our knowledge of the sequence of the low molecular weight myelin basic proteins, we propose to determine the sequence of the 17K protein and characterize the effect of the insertion upon both its structure and encephalitogenic activity. We will also determine the ability of the insertion to produce humoral antibody and use antibody specific for the 17K insertion to identify (by immunoblot) related proteins in myelin and to study the location and appearance of 17K-related proteins in the mouse during development.